1. Field of the Invention
The present invention relates to radio frequency (RF) interference cancellation, and more particularly, to RF interference cancellation in multicarrier transmission systems.
2. Description of the Related Art
Bi-directional digital data transmission systems are presently being developed for high-speed data communication. One standard for high-speed data communications over twisted-pair phone lines that has developed is known as Asymmetric Digital Subscriber Lines (ADSL). Another standard for high-speed data communications over twisted-pair phone lines that is presently proposed is known as Very High Speed Digital Subscriber Lines (VDSL).
The Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, has finalized a discrete multi tone based approach for the transmission of digital data over ADSL. The standard is intended primarily for transmitting video data and fast Internet access over ordinary telephone lines, although it may be used in a variety of other applications as well. The North American Standard is referred to as the ANSI T1.413 ADSL Standard (hereinafter ADSL standard). Transmission rates under the ADSL standard are intended to facilitate the transmission of information at rates of up to 8 million bits per second (Mbits/s) over twisted-pair phone lines. The standardized system defines the use of a discrete multi tone (DMT) system that uses 256 xe2x80x9ctonesxe2x80x9d or xe2x80x9csub-channelsxe2x80x9d that are each 4.3125 kHz wide in the forward (downstream) direction. In the context of a phone system, the downstream direction is defined as transmissions from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user). In other systems, the number of tones used may be widely varied. However when modulation is performed efficiently using an inverse fast Fourier transform (IFT), typical values for the number of available sub-channels (tones) are integer powers of two, as for example, 128, 256, 512, 1024 or 2048 sub-channels.
The ADSL standard also defines the use of a reverse signal at a data rate in the range of 16 to 800 Kbit/s. The reverse signal corresponds to transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term ADSL comes from the fact that the data transmission rate is substantially higher in the downstream direction than in the upstream direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over telephone lines.
Because both downstream and upstream signals travel on the same pair of wires (that is, they are duplexed) they must be separated from each other in some way. The method of duplexing used in the ADSL standard is Frequency Division Duplexing (FDD) or echo canceling. In frequency division duplexed systems, the upstream and downstream signals occupy different frequency bands and are separated at the transmitters and receivers by filters. In echo cancel systems, the upstream and downstream signals occupy the same frequency bands and are separated by signal processing.
ANSI is producing another standard for subscriber line based transmission system, which is referred to as the VDSL standard. The VDSL standard is intended to facilitate transmission rates of at least 12.98 Mbit/s and up to 51.92 Mbit/s or greater in the downstream direction. To achieve these rates, the transmission distance over twisted-pair phone lines must generally be shorter than the lengths permitted using ADSL. Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a similar system, which is referred to as Fiber To The Curb (FTTC). The transmission medium from the xe2x80x9ccurbxe2x80x9d to the customer premise is standard unshielded twisted-pair (UTP) telephone lines.
A number of modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). Most of the proposed VDSL/FTTC modulation schemes utilize frequency division duplexing of the upstream and downstream signals. Another promising proposed VDSL/FTTC modulation scheme uses periodic synchronized upstream and downstream communication periods that do not overlap with one another. That is, the upstream and downstream communication periods for all of the wires that share a binder are synchronized. With this arrangement, all the very high speed transmissions within the same binder are synchronized and time division duplexed such that downstream communications are not transmitted at times that overlap with the transmission of upstream communications. This is also referred to as a (i.e. xe2x80x9cping pongxe2x80x9d) based data transmission scheme. Quiet periods, during which no data is transmitted in either direction, separate the upstream and downstream communication periods. For example, with a 20-symbol superframe, two of the DMT symbols in the superframe are silent (i.e., quite period) for the purpose of facilitating the reversal of transmission direction on the phone line. In such a case, reversals in transmission direction will occur at a rate of about 4000 per second. For example, quiet periods of about 10-25 xcexcs have been proposed. The synchronized approach can be used a wide variety of modulation schemes, including multi-carrier transmission schemes such as Discrete Multi-Tone modulation (DMT) or Discrete Wavelet Multi-Tone modulation (DWMT), as well as single carrier transmission schemes such as Quadrature Amplitude Modulation (QAM), Carrierless Amplitude and Phase modulation (CAP), Quadrature Phase Shift Keying (QPSK), or vestigial sideband modulation. When the synchronized time division duplexed approach is used with DMT it is referred to as synchronized DMT (SDMT).
A common feature of the above-mentioned transmission systems is that twisted-pair phone lines are used as at least a part of the transmission medium that connects a central office (e.g., telephone company) to users (e.g., residence). It is difficult to avoid twisted-pair wiring from all parts of the interconnecting transmission medium. Even though fiber optics may be available from a central office to the curb near a user""s residence, twisted-pair phone lines are used to bring in the signals from the curb into the user""s home or business.
Although the twisting of the twisted-pair phone lines provides some protection against external radio interference, some radio interference is still present. As the frequency of transmission increases, the radio interference that is not mitigated by the twisting becomes substantial. As a result, the data signals being transmitted over the twisted-pair phone lines at high speeds can be significantly degraded by the radio interference. As the speed of the data transmission increases, the problem worsens. For example, in the case of VDSL signals being transmitted over the twisted-pair phone lines, the radio interference can cause significant degradation of the VDSL signals. This problematic radio interference is also referred to as radio frequency noise.
The undesired radio interference can come from a variety of sources. One particular source of radio interference is amateur (or ham) radio operators. Amateur radios broadcast over a wide range of frequencies with significant amount of power. The amateur radio operators also tend to change their broadcast frequency quite often, for example, about every two minutes. Another source of radio interference is AM radio transmissions by radio stations which broadcast over a wide range of frequencies. With high speed data transmission, the radio interference (noise) produced by various sources can significantly degrade the desired data signals being transmitted over twisted-pair phone lines.
Consequently, the problem with using twisted-pair phone lines with high frequency data transmission rates, such as available with ADSL and VDSL, is that radio interference becomes a substantial impediment to a receiver being able to be properly receive transmitted data signals. Thus, there is a need to provide techniques to eliminate or compensate for radio interference.
Broadly speaking, the invention pertains to radio frequency (RF) interference cancellation techniques that effectively estimate RF interference to transmitted data signals being received using a frequency domain model for the RF interference, and then remove the estimated RF interference from the received data signals. The invention also pertains to improved techniques for digitally filtering multicarrier modulation samples to reduce sidelobe interference due to the RF interference.
The invention can be implemented in numerous ways, including as an apparatus, system, method, or computer readable media. Several embodiments of the invention are discussed below.
As a method for mitigating radio frequency (RF) interference in a multicarrier modulation system, one embodiment of the invention includes the operations of: obtaining frequency domain data associated with a frequency band; identifying a restricted frequency sub-band within the frequency band; estimating a frequency of the RF interference within the restricted frequency sub-band; estimating the RF interference in accordance with a frequency domain model for the RF interference and the estimated frequency of the RF interference; and thereafter removing the estimated RF interference from the frequency domain data.
As a method for mitigating radio frequency interference in a multicarrier modulation system, another embodiment of the invention includes the operations of: identifying AM radio interference to the multicarrier modulation system, estimating a frequency of the AM radio interference, and disabling certain frequency tones of the multicarrier modulation system adjacent to the estimated frequency of the AM radio interference from carrying data during the data transmission, these operations occur prior to data transmission. Thereafter, during or following data reception, the invention also includes the operations of estimating the AM radio interference in accordance with a frequency domain model for the AM radio interference and the estimated frequency of the AM radio interference, and removing the estimated AM radio interference from the frequency domain data on those of the frequency tones of the multicarrier modulation system that carry data.
As a method for digitally filtering multicarrier modulation samples to reduce sidelobe interference from a radio frequency (RF) interferer, the multicarrier modulation samples occur at predetermined frequency tones and form a multicarrier modulation symbol, an embodiment of the invention includes the operations of: receiving x samples of a multicarrier modulation symbol and y samples of a cyclic prefix associated with the multicarrier modulation symbol, the y samples of the cyclic prefix preceding the x samples of the multicarrier modulation symbol; discarding an initial portion of the y samples of the cyclic prefix associated with the multicarrier modulation symbol; storing a remaining portion of the y samples of the cyclic prefix associated with the multicarrier modulation symbol; retaining a first portion of the x samples of the multicarrier modulation symbol without modification; and modifying a second portion of the x samples of the multicarrier modulation symbol in accordance with the stored samples of the remaining portion of the y samples of the cyclic prefix and predetermined multiplication coefficients.
As a receiver for a multicarrier modulation system, an embodiment of the invention includes: an analog-to-digital (A/D) converter, a multicarrier demodulator operatively connected to the A/D converter, and a digital RF interference canceller operatively coupled to the multicarrier demodulator. The A/D converter receives analog signals that have been transmitted to the receiver over a transmission media and converts the analog signals to digital time domain signals. The multicarrier demodulator receives the digital time domain signals and converts the digital time domain signals into digital frequency domain data. The digital RF interference canceller mitigates the effect of RF interference on the digital frequency domain data by modeling the RF interference in accordance with a frequency domain model. Preferably, the digital time domain signals include a plurality of multicarrier modulation symbols carrying data, with each of the symbols also including a guard band, and the receiver further includes a cyclic prefix removal and windowing processor operatively connected between the A/D converter and the multicarrier demodulator. The cyclic prefix removal and windowing processor performs a time domain windowing operation on the symbols.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.